/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineGroupsInterface.h
Promote an existing Admin to be the new Owner and simultaneously demotes the context user to Admin. Fails if the new owner is not currently an Admin of the group. Also fails if the new owner cannot own the group (due to owning too many other groups, as determined by the namespace config, perhaps).
void TransferGroup ( const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & ContextUserId, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & GroupId, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & NewOwnerId, const [FOnGroupsRequestCompleted](API\Plugins\OnlineSubsystem\Interfaces\FOnGroupsRequestCompleted) & OnCompleted )